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Grow yeast experiment

You will need:

 
  • 4 packets of dry yeast
  • 4 water bottles, chilled in the fridge (we use Thank You Water, a social enterprise that works to get clean water & sanitation to people in need)
  • 4 balloons
  • 1 funnel.
  • 1 spoon.
  • 1 large jug.
  • 4 measuring cups.
  • 4 thermometers (one will do if you don't have a class set).
  • Sugar.
  • Access to boiling water plus adult supervision.
  • 1 stopwatch.
  • A pen to mark the water temperature on each water bottle during the experiment.
  • A shelf to leave the science experiment to run.
  • A notebook for your observations.

    Copyright notice
Yeast growth experiment
Yeast growth experiment in progress; the bottle on the right has warmer water

Instructions

 
  1. Pour out the 4 chilled water bottles into the large jug and discard the rest of the water (maybe water your school garden!)
     
  2. Carefully measure out the water into the four measuring cups as per the measurements below;

    Cup 1 - 200mL of chilled water

    Cup 2 - 150mL of chilled water

    Cup 3 - 100mL of chilled water

    Cup 4 - 50mL of chilled water
     
  3. With an adult, boil a jug of water and then top up cups 2, 3 and 4 so that they too have 200mL of water as per cup 1. You will be testing the effect of temperature on the growth of yeast by measuring how much gas is released by the yeast under 4 different temperature conditions (variable testing). 
     
  4. Use the thermometers to take a measurement of the water temperature in each cup (write this in your notebook).
     
  5. Using a funnel, carefully pour each cup of water into the four separate water bottles. Use the pen to mark the starting temperature of each water bottle.
     
  6. Add a spoonful of sugar per water bottle and then swirl the bottle to dissolve the sugar.
     
  7. Add a yeast packet into each bottle and quickly stretch a balloon of the opening of each bottle.
     
  8. Start the stop watch and take notes of when each balloon rises!

    Yeast growth experiment with 4 bottles fille dwith yeast media and balloons on the bottle openings to catch the carbon dioxide released
    4 yeast growth experiments started, showing a distinct change already!
     
  9. OPTIONAL: you could also keep the each bottle in the yeast experiment at the same temperature and vary the amount of sugar added instead.

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What is going on?

Your experiment was testing the effect of water temperature on the growth of yeast. Yeast are egg-shaped microscopic cells of fungi which are dormant whilst kept in dry and cool conditions. However, yeast will rapidly divide once exposed to water and a sugar in ideal temperatures. In the right temperature, yeast cells will change the sugar into glucose by using the water plus as enzyme catalyst (invertase). Once the yeast has converted the sugar to glucose fermentation can then occur to produce carbon dioxide and ethanol as per the equation below;

Glucose ⟶ Ethanol + Carbon dioxide

which can be written as...

C6H12O6(aq)  ⟶ 2C2H5OH(aq) + 2CO2(g)

In your experiment you were trapping the carbon dioxide released during fermentation process. The more active the yeast, the more carbon dioxide the yeast produced! In your experiment the different water temperatures will have produced different results as some water may been too hot for the yeast to survive where as other bottle may been too cold. By introducing a variable to test in your experiment, you're doing real science! The following list of temperatures is worth keeping in mind when assessing your results:

  • 55° C – 60° C
    Yeast cells die (also known as the thermal death point).
     
  • 41° C – 46° C
    Ideal temperature of water for dry yeast being reconstituted with water and sugar.
     
  • 4° C
    The temperature of a fridge - yeast will be too cold to work properly.

Yeast is used to make bread rise and to ferment beer. There are many different species of yeast, but the one most commonly used in cooking and baking is called Saccharomyces cerevisiae, which is also known as brewer's yeast.

yeast-tandaco-35g-packet-5-sachet 600 x 600px
Yeast we used for our science experiment.

Yeast can break down many types of simple carbohydrates (monosaccharides) however they cannot break down complex carbohydrates such as starch. This means that extra enzymes are needed to break down starch into sugars that the yeast can use, for example during beer production we use enzymes from germinating barley to do this.

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